Inostrancevia vs Utahraptor: Utahraptor was slightly heavier than Inostrancevia. Inostrancevia had formidable jaws (with long upper canines), but Utahraptor had a more diversified offense (decent jaws; slashing kicks). Inostrancevia had decent mobility, but probably not enough to avoid a barrage of kicks from Utahraptor. Close fight, could go either way, but edge to Utahraptor.

Inostrancevia vs Hyaenodon gigas: Hyaenodon gigas was slightly heavier than Inostrancevia. Hyaenodon probably had greater agility, but Inostrancevia likely had the more damaging bite (with its long teeth). Edge to the slightly larger Hyaenodon gigas.

Inostrancevia vs Andrewsarchus: Andrewsarchus (according to conservative top-estimates) weighed over a ton, and was more than double the weight of Inostrancevia. Inostrancevia had upper teeth similar to a saber-tooth cat, and Andrewsarchus had a long set of jaws with crushing power. Andrewsarchus was probably the more agile animal, but its size advantage is what grants it the victory over Inostrancevia. Andrewsarchus wins.

Inostrancevia vs Entelodont: The largest entelodont (Daeodon) was more than double the weight of Inostrancevia. Entelodont had strong jaws, sharp tusks, & tough hide. Insotrancevia would have had a difficult time subduing Entelodont with an ambush, and face-to-face would have been an uphill battle. Entelodont wins.

Inostrancevia vs Postosuchus: Inostrancevia weighed about 2/3rd as much as Postosuchus. Inostrancevia was one of the early saber-toothed animals. Postosuchus' skull resembled that of some theropods, and it was designed to tear flesh with its sharp, serrated teeth. Postosuchus had osteoderms (bony plates) along its back that afforded it some protection from attack, but the jaws & teeth of Inostrancevia were designed to penetrate tough hides. Inostrancevia likely had the advantage in agility, but Postosuchus' powerful bite would cause more damage to Inostrancevia than the other way around. The time it would take for Inostrancevia to bite through the ostoederms would give Postosuchus time to retaliate with its own bite. A parity fight would favor Inostrancevia, but a Postosuchus with a 50% weight advantage will be favored. Postosuchus wins.

Inostrancevia vs Smilodon: Inostancevia had a slight weight advantage over Smilodon populator, but its relatively short legs made it somewhat less agile than the saber-tooth cat. Smilodon was powerful & muscular, and was well-adapted to wrestling adversaries into compromising positions with its forepaws (after which it could deliver a killing bite with its long upper canines). Smilodon fatalis, which was smaller than Smilodon populator, would have had a decent chance to defeat Inostrancevia (Smilodon gracilis would have been too small). Smilodon populator wins.

Postosuchus vs Andrewsarchus: Postosuchus reached almost 70% of Andrewsarchus' weight. Both animals had formidable jaws, and Postosuchus could rear into a bipedal stance. Postosuchus was somewhat protected by osteoderms (bony growths) along its back, but the larger Andrewsarchus would have been able to exploit unprotected areas on its body with its large jaws & better agility. Postosuchus had jaws designed to slice, and Andrewsarchus had jaws designed to crush. I would probably favor Postosuchus at parity, but not at normal weights. Andrewsarchus wins.

Postosuchus vs Smilodon: Only the Smilodon populator would have a chance in this matchup, and it would reach about 60% of Postosuchus' weight. Smilodon would have the advantage in agility, and would need to leap onto (or grab onto) an area of Postosuchus that enabled it to avoid a counter-attack from those dangerous jaws. Postosuchus had armor-like hide along its back, but the Smilodon's sabers would have been able to sink into its neck if it got into a favorable position. Close fight, but the Smilodon was accustomed to tackling large, dangerous prey and should prevail most of the time. Edge to Smilodon.

Andrewsarchus vs Hyaenodon gigas: These 2 animals had similar attributes (long, bone-crushing jaws), but Andrewsarchus was twice as heavy. Close at parity; not so close at these weights. Andrewsarchus wins.

Andrewsarchus vs Utahraptor: Andrewsarchus was twice as heavy as Utahraptor. Utahraptor had sharp, kicking claws & a decent bite, but Andrewsarchus had a huge set of jaws designed to crush bone. Andrewsarchus would need to take the initiative & land a good bite before being slashed by multiple kicks, and probably had the size to pull this off more times than not. Andrewsarchus wins.

Therizinosaurus vs Megatherium: Megatherium (giant ground sloth) was somewhat heavier than Therizinosaurus, but both animals had similar offensive attributes (upright stances, taller than a giraffe, swiping forelimbs). Therizinosaurus' claws were very long, but wouldn't have easily breached the tough hide (pebble-like pieces of bone under the fur that acted like armor) of Megatherium. Therizinosaurus wouldn't have held up well to the swiping claws of the larger Megatherium, and wouldn't be favored in this battle. Megatherium wins.

Phorusrhacos vs Deinonychus: Phorusrhacos weighed about twice as much as Deinonychus. Phorusrhacos was about the size of an ostrich, and had a deadly beak to bite & strike with. Deinonychus had jaws & teeth, and had sickle-shaped claws to kick & slash with. If Phorusrhacos had the ability to kick, it would give it an added weapon to its arsenal. The problem faced by Phorusrhacos was being able to keep Deinonychus from leaping upon in, sinking in its claws, and biting it with its jaws. The giant bird wouldn't have an answer for that particular attack, and would need to keep the dinosaur at bay with a combination of a snapping beak & repeated kicking. If the Phorusrhacos did not kick, it would lose to the Deinonychus more times than not. If the Phorusrhacos did kick (which is likely based on modern upright-walking birds), it would have a decent chance to injure the Deinonychus before it had a chance to leap upon the bird. Even though its outweighed, its ability to leap onto an opponent is huge for Deinonychus. Close to 50/50.

Gastornis vs Deinonychus: Gastornis was about 6.5 ft tall, and weighed almost twice as much as Deinonychus (although some heavier estimates exist) . Gastornis had a huge head, a stout beak with a hooked tip (good for ripping flesh & crushing small bones), & strong, clawed legs. Like the Phorusrhacos, Gastornis may have kicked adversaries in a conflict. Deinonychus was well-armed with kicking claws & a decent bite, and would have been able to leap upon Gastornis for a close-quarters attack. However, the massive beak of Gastornis would have caused serious injury to Deinonychus once it got in range. A lot of this outcome depends on whether or not Gastornis was an active kicker, and how big this animal actually got. I would favor Gastornis as long as it kicked & weighed at least double the Deinonychus' weight.

Entelodont vs Smilodon: The largest entelodont (Daeodon) weighed almost 2 1/2 times more than Smilodon populator. With tough hide, powerful jaws, & sharp tusks, Entelodont was a formidable adversary. Smilodon was a very muscular cat that was adept at wrestling down large, dangerous animals, but Entelodont would have been a handful without an ambush. Smilodon might succeed on occasion, but will be driven away by Entelodont most of the time. Entelodont wins.

Entelodont vs Utahraptor: The largest Entelodont weighed about twice as much as Utahraptor. The Entelodont would have used its strong jaws to bite & its dangerous tusks to slash. Utahraptor would have kicked with its lower claws to slash, and used its jaws to bite. The Entelodont would have been heavy enough to charge in and knock Utahraptor to the ground, and its larger size would have enabled it to dish out more damage than it would have taken. Entelodont wins.

Utahraptor vs Hyaenodon gigas: These animals weighed about the same. Utahraptor was a dangerous foe that used slashing kicks as its primary weapon. It also had a decent bite (not on par with large theropods, but still decent). Hyaenodon had a huge set of jaws with bone-crushing power, but would have had a difficult time dealing with Utahraptor's savage kicks. Both can win, but slight edge to Utahraptor.

Utahraptor vs Smilodon: Smilodon populator will weigh about 80% of Utahraptor's weight. Smilodon populators had robust, muscular bodies (built almost bear-like), and were adept at tackling large prey items to the ground and finishing them by delivering a bite into a soft-tissue area with their deadly upper canines. Utahraptor was a dangerous foe that used slashing kicks as its primary weapon. It also had a decent bite (not on par with large theropods, but still decent). The Smilodon's quickness & ability to leap upon its prey would be its meal ticket in this battle. The cat would leap onto the Utahraptor and likely bowl it over or drag it down without too much trouble most of the time. There's always the chance the Smildon could get kicked as it rushed in, and shift things quickly into Utahraptor's favor. On most occasions I see the Smilodon closing the distance & latching onto Utahraptor, and the theropod will have trouble landing kicks & bites as it gets pulled down. Smilodon's bulk means it wasn't as agile or maneuverable as, let's say, today's big cats, but it had a high enough level of these traits to utilize effectively against Utahraptor on most occasions. Smilodon fatalis & Smilodon gracilis would have been to small to prevail consistently against Utahraptor. Close fight, but Smilodon populator wins.

Arctotherium vs Megatherium: Megatherium was over 2 1/2 times heavier than Arctotherium (giant short-faced bear). Arctotherium would have a hard time penetrating the tough hide of Megatherium with its teeth & claws, and it wouldn't have been able to avoid getting hit by the giant sloth's huge, clawed forelimbs. The Megatherium simply has too much size here. Megatherium wins.

Expertise

Questions regarding animal conflicts within realistic or unrealistic settings are welcome; my strength lies in medium-to-large species. Small animals (including birds of prey), prehistoric animals, sea creatures, and domestic dog breeds are usually within my scope, but to a lesser degree. I can't confidently answer hypothetical questions about human vs animal, arachnids, insects, or amphibians, but I am willing to field them nonetheless.

Experience

From a young age, I have been interested in animals. Starting with the original Mutual of Omaha's Wild Kingdom and World Book Encyclopedias, I have seen many animal shows and documentaries and have read multiple books on the subject. I have a solid understanding of the physiology of many animals and interspecies conflict in general.